The invention pertains to the detection of mechanical defects within substrates covered with a protective coating which is transparent to visible light. In particular it is designed to detect the presence of cracks in the semiconducting substrate of solar cells.
It is known that solar cells include a functional or effective portion composed of a semiconducting layer and usually made of silicon (this material having an absorption spectrum centered on that of the Sun), adapted to generate electric current by photovoltaic effect, from incident photons produced by the Sun. In order to protect this functional portion against external agents, it is generally covered with a clear external coating, called cover-glass. Particularly in the case of solar cells which are part of solar generators designed for use in space on spacecrafts, it is important to be able to check the good operating condition of these cells, namely to check these cells' capability to generate electric energy: it is therefore necessary to be able to detect possible mechanical defects (such as cracks), in the semiconducting layers.
Likewise, in the case of satellites placed on a stationary orbit (i.e. telecommunications, air navigation monitoring, satellite-to-receiver telecasting, etc.) one knows optically reflective coatings to be applied onto radiating surfaces: these coatings are characterized by a high infrared emissivity and a low solar absorption ratio. Among these coatings, one knows rigid Optical Solar Reflectors (abbreviated as OSRs) which are made of a coating that is clear under solar rays and which are coated on their back face with a reflective silver layer constituting the functional portion of the solar reflector. In this case too, it is important to be able to predict on ground the future performance of these solar reflectors, and thus to be able to detect any possible mechanical defects (such as cracks) in the reflective silver coatings.
Whether it be solar cells or rigid optical solar reflectors, the presence of a mechanical defect practically causes the defective component to be rejected. However, since all of the mechanical defects do not equally affect the performances of the elements considered, it is interesting to be able to differentiate between critical mechanical defects justifying a reject and minor mechanical defects which little affect the performances and which do not justify, per se, a reject. In particular, mechanical defects such as cracks within the clear coating of solar cells or optical solar reflectors are much less objectionable than geometrically similar mechanical defects found in the semiconducting or reflective coatings respectively.
German Patent 3,012,162 already describes a device designed for the inspection of photovoltaic effect semiconducting components taken one by one in a small diameter beam of light. There is also known a solar cell network inspection device using an infrared microscope, as described in article "A portable, X-Y translating, infrared microscope for remote inspection of photovoltaic solar arrays" by S. E. FORMAN and J. W. CAUNT, MIT Lincoln Laboratory dealing with a lecture given at The International Symposium for Testing and Failure Analysis, Los Angeles, Calif., on Oct. 27, 1980.
As the case may be, these devices present several disadvantages, such as large dimensions, mandatory operation in infrared, complexity of operation, high cost, etc. Furthermore, they do not contemplate discriminating the mechanical defects existing in the semiconducting layer from those existing in the clear coat.
For information, in an entirely different field, U.S. Pat. No. 3,676,008 may also be mentioned as it pertains to automated optical inspection of such bodies as glazed tiles; however, its teachings do not provide the means to offset the aforementioned disadvantages.
This is the reason why, in practice, the inspection of the solar cells already installed within a satellite solar generator, in order to check for defects such as cracks in the silicon substrate or in the clear protective coat, breaking of the interconnecting pieces or welding defects, is performed either with unaided eye or with binoculars under glancing light, which sometimes leads to subjective conclusions depending notably on the state of alertness of the operator. It should be noted that this human inspection is time-consuming (averaging approximately 10,000 cells inspected weekly, with a satellite such as TV SAT comprising 40,000 solar cells) and tiresome (since the inspection is performed in the dark).
Accordingly, what is needed is a defect detecting device on a visual principle designed for the inspection of solar cells, as well as optical solar reflectors, preferably under visible light (thus simplifying the operation), while allowing simultaneous discrimination between mechanical defects found in the functional layer and mechanical defects found in the clear protective coating. The device is fast and readily automated to large dimension component networks to be inspected, while remaining altogether simple, reliable and economical. The device is also preferably adapted to detect the orientation of the possible defects.